The liquid dye laser is currently used as a source of laser excitation or amplification in applications of laser enrichment, such as shown, for example, in U.S. Pat. No. 3,944,947. In such applications, there are at least three principal objectives for the laser oscillator or amplifier, namely high energy in each laser pulse, high repetition rate, and an output beam of laser radiation of high optical quality and low divergence. In these applications output powers of several hundreds of watts at pulse rates of several hundreds of pulses per second in combination with an output beam as close to defraction limited as possible are desired goals. One of the important advances in laser systems in achieving these objectives has been the transverse pump laser as shown in U.S. Pat. No. 3,740,665, wherein the optical axis is transverse to the flow direction of the fluent laser material, typically a laser dye solution. This permits a rapid replenishment of dye into the region of the optical axis to replace expended dye, increasing both power and repetition rate.
To some extent both power and repetition rate can be augmented by increasing the level of applied exitation to the optical axis and by increasing the flow velocity of the fluent laser material. Excessive heating and breakdown of fluid dynamic flow characteristics are limiting factors here as well as turbulence due to heating effects. In addition, the variation in refractive index throughout the fluent laser material produced by temperature variations greatly degrades the beam quality.
One source of such temperature gradients and turbulence is the heating of the window seal region and of the flow channel walls upstream of the lasing region by the absorption of reflected lamplight or stray dye fluorescence light produced within the flow channel by the excited dye solution. Another cause of heating is the absorption by the walls in the window seal region of flashlamp light reflected from the interface formed between the window and the liquid dye solution. A third cause of heating of the flow channel walls is the absorption of light emanating directly from the flashlamp. For flow channels in which the flow upstream of the lasing region is not strictly laminar, temperature and the associated refractiveindex gradients generated as the liquid moves past the channel wall may appear in the lasing region during subsequent pulses. These gradients severely degrade laser beam quality, further reduce laser output energy, which in turn increases the amount of fluorescence available to cause further channel wall heating.
Heating of the flow channel walls upstream by the absorption of stray fluorescence light may be considerably reduced in the manner shown in U.S. Pat. Application Ser. No. 852,224, assigned to the assignee of this invention, and now U.S. Pat. No. 4,178,565.